What is a planet and what is a star?

A planet usually refers to a celestial body that surrounds a star without giving off light. Stars are spherical luminous plasmas condensed by gravity.

Specific introduction:

Planet:

A planet usually refers to a celestial body that surrounds a star without giving off light. Its revolution direction is often the same as the rotation direction of the surrounding stars. Generally speaking, the planet needs to have a certain mass, and the mass of the planet should be large enough to approximate a sphere, so that nuclear fusion reaction can occur like a star.

Stars:

The star is a spherical luminous plasma condensed by gravity, and the sun is the closest star to the earth. Almost all the stars that can be seen at other nights on earth are in the Milky Way, but because of their long distance, these stars seem to be just fixed luminous points.

Historically, those prominent stars have formed constellations and clusters, and the brightest stars have their own traditional names. The catalogue of stars compiled by astronomers provides many standards for the naming of different stars.

Extended data:

Number of stars:

Astronomers have always had different estimates of the number of stars in the universe. One of the most famous sayings is a conjecture put forward by American astronomer carl sagan in his book Hundreds of Billions. He believes that there are 654.38+00 billion galaxies in the universe, and each galaxy has 654.38+00 billion stars.

Astronomers further speculate that the number of stars in each galaxy is about one trillion times that of 654.38+000 billion. After analyzing the light intensity from galaxies, American astronomer Peter Van Dokun and astrophysicist Charlie Conroy think that it is about 3X 1023.

Looking for exoplanets:

1, astrometry:

Astrometry is the oldest method to find extrasolar planets. This method is to accurately measure the position of a constant star in the sky and observe how the position changes with time.

If a star has a planet, the gravity of the planet will cause the star to move in a tiny circular orbit. In this way, stars and planets revolve around their same center of mass. Because the mass of a star is much larger than that of a planet, its orbit is much smaller than that of a planet.

2. Apparent velocity method (radial velocity):

The apparent velocity method uses the star to move in a tiny circular orbit under the action of planetary gravity, and its goal is to measure the speed at which the star moves towards or away from the earth. According to the Doppler effect, the apparent velocity of a star can be inferred from the movement of its spectral lines.

3. Transfer method:

When the planet moves in front of the star, the light of the star will decrease accordingly. The degree of light attenuation depends on the size of stars and planets. Take star HD 209458 as an example, its light input decreases by 1.7%. Astronomers discovered the planet HD 209458b of the star HD 209458 by transit method.

4, pulsar timing method:

The existence of planets can be inferred by observing the signal period of pulsars. Generally speaking, the rotation period of a pulsar, that is, its signal period is stable. If the pulsar has a planet, the pulsar signal period will change.

5, gravity microlens method:

The method of discovering planets by using gravitational lens effect. For example, the planet OGLE-2005-BLG-390Lb was discovered in this way.

References:

Baidu encyclopedia-planet

References:

Baidu encyclopedia-star